Post by Lee Martin on Dec 26, 2013 15:32:11 GMT -5

Thanks Steve. Those are leveling V-blocks for our mill, not mold blanks.

My dad cut a few bullet molds in the 90's just to see if he could. They turned out really well but in the end it wasn't worth the time and effort. Folks like NEI get up in the morning to do that type of work.

However there is a 10mm mold I want and we may make our own. If we do I'll post pictures.

Post by Lee Martin on Dec 26, 2013 15:39:21 GMT -5

The receiver’s inside diameter is 0.877” post honing. Between centers we rough turned the bolt to 0.883”:

I’ll talk in detail about our action’s design as the thread progresses. Without jumping ahead too much though it’s similar to the Hall Manufacturing BR. True to the Hall our bolt body and locking lugs are the same diameter. This not only removes the need for broaching behind the raceway but we believe it increases rigidity. That said we’ve built a few actions with exposed lugs (aka Remington style) with great success. The strengths and weaknesses of each will be covered shortly.

We hold to ridiculously low tolerances on the bolt; some say too low in fact. Ultimately we aim for 0.001” difference between the bolt and receiver. Most benchrest actions are 0.0017” – 0.0025” with Borden being the lowest. Their patented bolt bumps net them <0.0015”. Comparatively speaking commercial actions are well over 0.005”. The Hall design allows us to get to a thousandth which is next to impossible on a broached receiver.

The key to this tight a fit is two-fold. One you need absolute concentricity and exact diameter end-to-end. Being out 0.0008” over the length will cause bind. We fit them that tight. Secondly the mating surfaces must be mirror smooth. Boring or lathe turning won’t get you there. Honing the inside of the action will however and our Sunnen dials to ten-thousandths. So what about the bolt? We purposely turned large in order to precision grind the final dimension. Using a Cincinnati No. 2 grinder we chucked the body. This machine is over 50 years old but still tables to 0.0002” over ten inches. I won’t even say what the modern machines yield but they’re impressive (and expensive).

Post by curmudgeon on Dec 26, 2013 16:10:50 GMT -5

Nice work Lee. I've built several rifles on the old HALL G action. Spent a day with him showing me the whole shop in his basement, fancy basement. ATB LEJWas sorry to see him discontinue the G action, But since I'm not building now guess it doesn't matter. Never felt confident to try and build an action, didn't have a grinder either, just surface grinder. Too old now. :-)

Post by Lee Martin on Dec 30, 2013 15:38:27 GMT -5

The Bolt Lugs____________________________________________

A bolt action is only as good as its locking lugs. Those little bumps absorb the case thrust, keep pressure moving forward, and thwart a bolt to the face. In view of the pressures generated by modern smokeless rifle cartridges this is no small task. 50,000 PSI is the norm and belted Magnums routinely exceed 60,000 PSI. Even the seemingly piss-ant 6 PPC pushes the envelope with certain tunes hitting 70,000 PSI. For that reason most benchresters won’t extract all they can from the round with conventional receivers. The competitive actions you see are much stronger.

One reason dad and I like the Hall style is the lack of side raceways. The bolt body is within a thousandth of the receiver behind the lug seats. Since there’s 0.001” total fit (or 0.0005” per side) beyond that you have ample seal if the case ruptures. A broached action has raceways with weaker tolerances; that room could allow some gas venting towards the back of the action. But all of this is a moot point if the lugs do their job.

Locking lug prowess is a function of shear strength, bolt thrust, and to a lesser degree bolt flex. Shear strength is the ability for a metal to resist sliding failure of the material. This resistance is parallel to the force applied. So with a bolt action the case head generates force perpendicular to the lug causing potential shear along the axis (parallel). Bolt shear strength is calculated by:

The only tough variable to derive is the arc segment. If you know the bolt’s radius you can use an arc cosine function to calculate “X”. I won’t bore you with the 4 or 5 equations it takes to do this. For our action X = 2.3412.

Once completed we’ll heat treat it to Rockwell 40 C scale. That’ll provide a yield tensile of 170,000 PSI. Plugging all of this into the above equation:

Lug Shear Strength = (2.3412*0.60*2*170,000)/2 = 238,802 PSI.

The potential for a lug to shear under load is due to bolt thrust. This calculation couldn’t be more straightforward. It involves knowing the chamber pressures and the inside diameter of the case head. Let’s use the 6 PPC for illustration. Lapua webs have an inside diameter of 0.365”.

Area = 0.1045 (3.14*(0.365/2)^2)

Bolt Thrust = Pressure * Area

6 PPC Bolt Thrust = 60,000 PSI * 0.1045 = 6,274 lbs/ square inch

Thus far I’ve covered the rearward force exerted on the bolt head. Bolt flex is another variable at play. When the lugs seat against the receiver they absorb a sudden amount of load. That load is primarily applied to the back of the lug itself. If 100% uniform mating is present between the lug and receiver bolt flex is slightly offset. If not an uneven force is applied along the axis.

To determine bolt flex we need to know the steel’s shear modulus of elasticity. For 4140 steel the value is constant at 11,500,000 lbs.

Bolt Flex = Bolt Thrust / (Shear Area * 11,500,000)

We figured the bolt thrust at 6,274 lbs.

The Shear Area = (Length of Arc Segment)*(Lug Length)*(Number of Lugs)

Shear Area = (0.723)*(.600)*2 = 0.8676

Bolt Flex = 6,274 / (0.8676*11,500,000) = 0.00067

Sounds impressive but what does it mean? Basically it gives us some semblance of how much the bolt flexes or sets back under load. The farther the travel, the more the brass is subjected to stretch. Using our action we see it’s just over a half ten-thousandth. For commercial receivers bolt flex is usually around two thousandths. Note from the above equations that bolt flex is reduced with long lugs and high shear area.

None of these equations are new. Nor are the results that surprising. I’ve only included them here to illustrate why I like the Hall design. By making the lugs the same diameter as the bolt we’re able to achieve greater shear area. And when combined with a long lug you get 1) High shear strength, and 2) Very low bolt flex.

Below is a picture of a Remington 700 bolt. Compare the height and length of the lugs to ours:

The differences may appear subtle. But adding a tenth or two of depth and a few hundredths in height significantly bolsters strength (as does increasing the diameter of the bolt body).

Post by Lee Martin on Dec 31, 2013 11:03:32 GMT -5

Lee - I know this is a labor of love but I'm just curious, how man man hours have you got into it so far? Any WAG on how many hours it will take to complete?

Hard to say but I'd guess 15 hours so far. To complete the entire action will likely be 40+ but that includes the scope base and rings (we make those as well out of aluminum). This is the 17th or 18th receiver my dad has built so he's got the process down pat.

Post by Lee Martin on Dec 31, 2013 11:18:07 GMT -5

Markbo....both very good questions so let me attempt to answer them.

2-Lug versus 3-Lug

The 2-lug or 3-lug debate started in the 1970s within the benchrest community. After 40 years no one has shown an accuracy advantage between the two. The original theory was the 3-lug better resists off-axis force. Decades later that has never translated into smaller groups. It's interesting speculation and should probably be pursued further in the lab. And as for strength there’s no material difference. A slight edge to the 3-lug maybe but the bearing areas are nearly the same (2 big lugs or 3 smaller lugs; the shear footprints are typically within 5 – 10% of one another).

Now some prefer the 3-lug's shorter bolt throw. Obviously they feature 60 degree swing versus 90 with the 2-lug. Reduced travel comes with a price however, namely harder bolt lift. With a 2-lug the cocking ramp is more gradual. With a 3-lug it’s a bit more abrupt resulting in an additional ~1 lb of force to cam. That’s minor in the field but enough to make bag positioning trickier on the line (ie, there’s a greater chance of rocking the gun on the rest). Bruce Thom introduced a cocking cam roller on the BAT 3-lug which aids bolt lift. But even with the upgrade they’re still harder than his 2-bump.

BAT 3-lug bolt:

The majority of competitive benchrest actions are 2-pattern. Examples include the Stiller Viper, Borden, Hall, and what I consider to be one of the finest commercial offerings, the Kelbly Panda. BAT offers 2 and 3-lug bolts and their sales are about equal.

You also asked about why we didn’t go with a short action, and in turn a shorter bolt. While compact implies high rigidity there’s a trade-off. Back when benchrest started actions were long and less rigid. The Mauser 98 is a great example of this. With the thumb cut-out and wide loading port gunsmiths sought to stiffen them. The three common ways they did were to: 1) Weld the magazine notch, 2) Cut, shorten, and re-weld the receiver, and 3) Sleeve the entire action. Two and three added the most rigidity. The problem with going short though is you lose bedding surface. Modern benchrest actions are so well engineered the rigidity delta between 9" and say 6.5" is negligible. The other downside to really short actions is you give up barrel shank and lug length. Think of it this way. The port has to be so long to accept the loaded round. With our action we used a 2.40” opening which is close to minimum on the PPC. I also wanted a tenon over 1.0” and locking lugs 0.600” in length. Coupling that with a 0.75” raceway and you’re close to 5.0”. Add a trigger and cocking cam and you’re at 7.0”. If you look at modern BR actions most fall between 8.0” and 9.0”. Again, that gives a generous bedding area, more than enough rigidity, ample barrel support, and room for three pillars if you bed that way. Even on glue-ins you’ll benefit from the longer receiver. Short tubes and heavy glued-in barrels make tune-harmonics a challenge.

Post by Lee Martin on Jan 6, 2014 13:15:21 GMT -5

Step 20- Recessing the Bolt_________________________________________

Before counter-sinking the bolt head we faced it with a single-point cutter:

The 6 PPC mics 0.440” at the rim and the web. We’ll go a hair bigger so the extractor can pivot the case. We also want to avoid rim pinch as it engages the lip in battery. We agreed on 0.444” for the recess width, 0.120” on depth.

A boring bar made the step:

Rough turned:

Dimensionally done. Later on we’ll surface polish the bolt face with a brass lap:

Post by Lee Martin on Jan 9, 2014 16:25:05 GMT -5

Throughout this thread I’ve name dropped guys that made benchrest what it is today. Their contributions span decades and vary in nature. Some improved the components, others refined shooting techniques, and a smaller subset formed the competitive bodies that govern the sport. Certain contributions can’t be recalled in passing though, their recognition should be standalone. So with that in mind I’ll start with Ralph Stolle.

Stolle, ~1981:

Ralph Stolle was born in Elizabeth, West Virginia in 1922 and worked as a printer around Washington DC. It put him in close proximity to the origins of organized benchrest and the matches that followed. A natural perfectionist, attention to detail consumed him. No surprise he quickly fell into benchrest and its compulsive demands. Those early years, the 1940’s and 50’s to be exact, entailed a lot of trial and error. Bullet forming, case design, barrel making, and stock bedding were among the challenges they faced. Receivers evolved as did the link between high precision and rigidity. Common ways to stiffen them included shortening commercial makes, welding recesses, and sleeving. In many respects it mirrored what was happening in the hot rod scene. Liquor runners and draggers found assembly line cars lacking. A decent base maybe, but their performance objectives weren’t being met by mainstream production. Heavier suspensions and hopped-up motors soon flourished and so did speed. Benchrest actions have a similar lineage.

Mike Walker of Remington fame put precision shooting on the fast track in 1950. The arrival of the .222 and Model 722 in ‘48 gave the sport a big lift. That potent duo shrunk groups but competitors wanted more of an edge. Other things like cartridge profile, barrel making, and bullets also advanced during this era. The focus soon shifted to improving actions and those that dabbled in this area are a proverbial who’s who. Ed Shilen and Clyde Hart immediately come to mind. But Ralph Stolle probably moved the needle more than anyone when it came to their design. Early on he tried a lot of combinations, mostly aluminum collared Remingtons. They worked but never satisfied his incessant drive to do better. No longer would he mend someone else’s product. He decided to start from scratch.

Mike Walker at the bench (~1952):

Stolle knew the success of any benchrest action was its ability to stabilize heavy target barrels. The consummate student of physics, he paid close attention to harmonics and more importantly deflection. Around that time (mid-1960’s) some marksmen applied calculus to actions and derived their rigidity. Understand these were all ground zero efforts. No shooting click had ever tried to squeeze this much precision from a long-gun. Every variable was under a magnifying glass, every angle was vetted.

A vintage benchrest rig, complete with Unertl glass:

In the late 1950’s another DC resident named Homer Culver began work on a benchrest receiver. Eventually they became close friends and were deeply connected to competitive benchrest. I’ll cover Culver later, for now just note the two influenced one another. The Culver action came first and featured an integral Unertl dovetail. Homer built a dozen or so but they were never marketed. Stolle on the other hand sought to meet a growing demand for competition receivers. Together they figured out some of the core processes involved in machining an action. Broaching, tooling, heat treatment, and cam inleting were among those tackled.

An example of a sleeved Remington Model 600:

The original Stolle action dates to 1967. Coined the Grizzly, it was a large all steel tubular design for the heavy-gun class. Length capped out at 15” and it weighed 6.5 pounds. The barrel was shrouded well past the tenon and its strongback trigger guard mated to four pillars. It was massively rigid and shot well but couldn’t make limit in the growing Varmint classes.

Like any new venture Stolle climbed the learning curve. His first actions were machined from annealed round stock and then hardened. Warpage and shrinking caused tolerances to shift after quenching. Ralph scraped them and switched to pre-hardened alloys, normally Rockwell 32 – 34 C 4140 stock. It was hell on tooling and increased production time but the dimensions held. The raceways alone required an hour per side to slot with a single-edge form cutter (he milled 0.001” per pass). End-to-end it took 25 hours to make a Grizzly and that didn’t include bolt fitting. Quite an investment but the returns were significant. The Grizzly’s vast forward breech provided off-the-barrel mounting points for long target scopes. It also added valuable area for bedding, thus shifting the balance to the gun’s centerline. Bolts were quite conventional, either a 722-length Remington or a custom version by Will Gardner of Burtonsville, MD. Gardner’s was “Rem-esque” but did use longer handles and more positive post-64 Winchester type extractors.

Stolle at the range, late 1960’s:

In 1971 Stolle introduced the Panda for the Varmint classes. Initially these were cylindrical but by ’74 he switched to a flat-bottomed pattern. Lightweight, rigid, and easy to bed, the Panda has become a cornerstone action within the sport. Made from 7075 T6 aluminum, Stolle was able to upsize the dimensions without compromising strength (aluminum is 1/3 the density of steel; 0.10 versus 0.28 pounds per cubic inch). Two chrome moly inserts are threaded and epoxied into the receiver. The rear segment forms the cocking cam, the front houses the locking seats and threaded ring.

Stolle Pandas:

1974 also brought forth the Polar. This big action was just an aluminum hulled, steel inserted Grizzly. At 5 pounds it could be HV eligible yet retained the dimensional benefits of its bear brethren. This chart illustrates the rigidity and bedding advantages of these models:

In late 1976 American Rifleman featured the Stolle and their review was favorable. At that point Ralph had produced around 100 receivers and hoped to expand production. In 1978 he retired from the print industry and moved from Seabrook, MD to his hometown of Elizabeth, WV. Availability didn’t increase much after the move but Stolle’s reputation did. His actions were well regarded and strong performers on the line. Then things took a turn for the worse.

Ralph Stolle in his Maryland based shop (1976):

In 1979 Ralph and his wife Louise traveled to George Kelbly’s range for an unlimited and heavy varmint event. While on-site in North Lawrence, OH he suffered a bad heart attack. Ralph was stubborn and asked not to be taken to the ER. He eventually caved and when admitted underwent heart bypass surgery. The Stolles stayed at Kelbly’s until he felt strong enough to return to West Virginia. He did but soon worried about access to his remote home. Let’s just say it wasn’t ambulance friendly. Enter George Kelbly. Stolle met Kelbly at the DuBois, PA Nationals in 1957. The two became long-time friends and even did some actions together. George, a construction contractor by day, built the Stolles a home on adjoining land. By 1981 Ralph was back in the shop and George went full-time into his rifle business (like Culver, I’ll feature Kelbly later in this thread). Unfortunately the union was short-lived. In 1982 Ralph suffered a fatal heart attack.

The Stolle action lives on thanks to George Kelbly and team. The Polar is gone, but the Panda and Grizzly II can be had in a variety of configurations. And in honor of his friend, “Stolle” is engraved on every Panda they sell.

After Ralph passed George gave half of the profits to his widow until her death. Benchrest has always been a gentleman sport; this gesture more than reinforces that notion.